The present invention relates to an enamel frit composition used for direct coating of enamel on a base material made of a metal (iron, stainless steel or the like).
Conventional enamel frit compositions for base materials made of metal (iron, stainless steel or the like) contain transition metal elements such as Co, Ni or the like which produce oxidation reactions with the base metals to form Fe-Co alloys or Fe-Ni alloys (dendrite crystal) during firing of the enamel and which adhere closely to the base metals owing to such chemical bonding.
During this firing adhesion process, however, the enamel includes a large amount of gases comprising hydrogen and oxygen gases generated by reaction between the metal and water in the course of firing, carbon dioxide gas generated by oxidation of the carbon present in the metals, and air captured therein. The enamel therefore suffers from the disadvantage of becoming porous.
Such enamel frit compositions are generally used in the form of a ground enamel. Conventional enamel can be obtained by spraying such ground enamel several times and then spraying several cover coatings. Since the need for several sprayings of ground enamel and cover coatings leads to complication of the overall manufacturing process, industry has recently shown a tendency to demand enamel that can be provided directly on a base metal in the form of a frit composition with resistance to chemical corrosion.
In order to obtain an enamel finish by direct coating without using any ground coating, a pretreatment is required in which nickel serving as an adhesive agent or an adhesion accelerator for a base metal is deposited on the surfaces of the base metal. In this pretreatment, a low carbon steel plate to be enamelled is washed with an acid so that the surfaces of the steel plate are strongly corroded by dissolving out 25 g/m.sup.2 or more of the iron on the surfaces of the steel plate, and 0.6 to 1.5 g/m.sup.2 of nickel is then deposited on those surfaces by using nickel treatment.
Such pretreatment with a nickel metal enables enamelling having a relatively good level of adhesion to be obtained by directly spraying a cover coating, without spraying any ground coating. The enamels obtained by this method, however, do not have a level of adhesion sufficient to allow them to be used in reaction vessels or the like which are used under harsh conditions.
In order to reduce the viscosity of an enamel during firing, direct ground coatings generally contain small amounts of an SiO.sub.2 component and large amounts of R.sub.2 O (wherein R denotes an alkali metal such as Na, K, Li or the like) and B.sub.2 O.sub.3, as well as an element such as Co, Ni, Mn or the like. The ground coatings therefore have the disadvantage that they have substantially no resistance to chemical corrosion owing to the nature of their composition.
Accordingly, it is an object of the present invention to provide an enamel frit composition used for direct coating which possesses none of the above-described disadvantages.